Slough House, which was not its real name — it didn’t have a real name — was openly acknowledged to be a dumping ground: assignment here was generally temporary, because those assigned here usually quit before long. That was the point of sending them: to light a sign above their heads, reading EXIT. Slow horses, they were called. Slough House/slow horse. A wordplay based on a joke whose origins were almost forgotten.
— Dead Lions (Mick Herron), Loc. 218–21